Die casting hollow articles by lost core process



y 1966 E. w. REARWIN 3,258,816

DIE CASTING HOLLOW ARTICLES BY LOST CORE PROCESS Filed Aug. 9. 1963 2 Sheets-Sheet 1 A TTORNEYS July 5, 1966 E. w. REARWIN 3,258,816

DIE CASTING HOLLOW ARTICLES BY LOST CORE PROCESS 2 Sheets-Sheet 2 Filed Aug. 9, 1963 United States Patent 3,258,816 DIE CASTING HOLLOW ARTICLES BY LOST CORE PROCESS Earle W. Rearwin, High Point, N.C., assignor to Adams- Millis Corporation, High Point, N.C., a cor oration of North Carolina Filed Aug. 9, 1963, Ser. No. 300,999 6 Claims. (Cl. 22-168) This invention relates to die casting and, more particularly, to a novel process for die casting a hollow article.

It is the general object of the present invention to provide a method for producing a hollow die cast article and a disposable core for use in practicing this method.

A more specific object of the present invention is to provide a method for producing a hollow die cast article in which the internal surface is held to a very close tolerance.

Another specific object of the invention is to provide a method of die casting hollow articles by the use of a disposable core which allows the molten metal to form an internal surface that can be held to close metallurgical specifications.

The drawings show preferred core structures used in accordance with the invention and such structures and the preferred methods of the invention will be described, but it will be understood that various changes may be made from the structures and methods disclosed, and that the drawings and description are not to be construed as defining or limiting the scope of the invention, the claims forming a part of this specification being relied upon for that purpose.

Of the drawings:

FIG. 1 is a perspective view of a disposable core so constructed as to be usable in a lost core process of the present invention;

FIG. 2 is a sectional view along line 2-2 of the core of FIG. 1. Also shown are the two halves of the die which when engaged as shown define a hollow die cavity in which a hollow article may be die cast according to the present invention;

FIG. 3 is a sectional view similar to FIG. 2 but showing the article after casting, the die halves being shown in broken lines;

FIG. 4 is a perspective sectional view of the die cast article of FIG. 3 and the core of FIG. 1 showing these members after they have been removed from the die casting machine but before the disposable core has been separated from the article; and

FIG. 5 is a perspective sectional view of the hollow die cast article after the core of FIG. 1 has been removed.

For the purposes of illustration, the lost core process of the present invention is shown as applied to the die casting of a hollow article such as an aluminum water jacket 10, best shown in FIG. 5. The shape of the hollow interior of this article dictates the shape of the disposable core 12 best shown in FIG. 1. It should be understood, however, that the present invention is not limited in its scope to the fabrication of articles of this particular shape, but, on the contrary, the method of this invention can be practiced in fabricating any die cast article having a hollow interior portion. Thus the article 10, and its associated core 12, are chosen to illustrate the advantages of the lost core process in a particular application. It will be noted that conventional die casting techniques are virtually incapable of offering an economically feasible solution to the problem of die casting a hollow article such as the one shown in FIG. 5.

Briefly stated, one method of practicing the lost core process of the present invention consists in a first step of providing a core 12 which'is preferably of zinc material and has an outer surface corresponding to the hollow interior of the aluminum water jacket 10. End portions 14, 16 of said core project outwardly beyond the inlet 15 and outlet 17 of the water jacket as best shown in FIG. 4. Secondly, this core is coated with a heat resistant shield 18 of Vermiculite or the like, and thirdly the insulated core is placed in the die casting cavity 20 of a die casting machine as shown in FIG. 2. The cavity 20 cor responds to the exterior dimensions of the aluminum water jacket 10 except for those parts 22, 24 of the die halves 26 and 28, which accommodate the respective core end portions 14 and 16. These elongated portions will be seen to support the core in proper relationship to the die halves 26 and 28 at least when these members are engaged. Casting is accomplished by introducing a molten metal such as aluminum into a runner and through a gate into the die cavity 20 according to conventional die casting practices which comprise the fourth step in practicing the hereindescribed method. Finally, the fifth step involves immersing the die cast article with its associated core in a salt bath at approximately 700 F. At this temperature, the zinc core will melt causing the Vermiculite heat shield to disintegrate and the melted zinc to settle to the bottom of the salt bath where it can be recovered for reuse if desired. The aluminum jacket 10 having a melting point somewhat above 1000 P. will be relatively unaffected by the salt bath which heats up the jacket and core structure in a very uniform manner.

Referring now with greater particularity to the steps outlined above, the first of these, that of providing the zinc core 12, will preferably entail the fabrication of a die cast or accurately machined core in order to assure that the surface thereof is at least as smooth as the tolerances desired in the inner surface of the hollow water jacket 10. Also in connection with the zinc core, it will be apparent that the diameter thereof, including the heat shield 18, may be adjusted to provide for anticipated shrinkage of the cast aluminum jacket 10. As mentioned, the zinc core 12 is preferably longer than the aluminum jacket 10 and the end portions 14 and 16 thereof are well suited to provide the required supporting means for pro-perly positioning the core during the actual die casting operation which comprises step four of the process described herein.

With reference to the heat resistant shield 18, the material mentioned is a type of mica consisting of many tiny flakes which are available commercially in this form under the mark Vermiculite. In practicing the method of the present invention, a solution of latex and water is first prepared and the mica-like flakes are introduced into this solution until a slurry is formed. By dipping the zinc core 12 into this slurry, a quantity of the Vermiculite will be deposited on the surface of the core to a thickness which is determined in part by the density of the slurry and in part by the length of time the core is so immersed.

This mica-like material has been found to be quite satis factory and presently it is the preferred form of the heat shield 18. However, any good insulating material might be adapted to be deposited on the core 12 so as to withstand the momentarily high temperatures of the molten aluminum without any appreciable heat transfer to the zinc core. The mica-like material used has proven to be capable of withstanding molten aluminum at some 1100 F. and its heat transfer properties are such that very little of this heat is allowed to reach the zinc core 12 which will begin to melt at about 650 F.

In addition, the thickness of the mica-like coating is such that it is incapable of maintaining the shape of the solidified core as the latter begins to melt in the salt bath alluded to previously. Thus, the heat shield 18 is automatically destroyed as the zinc core is melted and the tiny flakes are carried out of the aluminum article 10 along with the a molten zinc to yield the hollow aluminum jacket of FIG. 5. It should perhaps be noted that the salt bath of step five in the above-described process is the preferred method of melting the zinc core because it avoids changes in the physical characteristics of the aluminum which might occur if a conventional heating oven were employed, and it also permits economical recovery of the molten zinc for subsequent use in fabricating other cores.

Turning now to a second, or alternative, method for practicing the lost core process of the present invention, it will be apparent that other types of cores might be employed if alternative means could be devised for destroying the same subsequent to the die casting step described above. In this vein, the presently preferred alternative method employs a packed sand core which is subsequently crumbled by any convenient means to yield a hollow die cast article such as the jacket of FIG. 5.

Thus, the first step in this second or alternative method consists in providing a packed sand core similar in shape to the Zinc core described heretofore for use in the fabrication of a jacket such as that shown in FIG. 5. The use of sand cores is not conventional in the die casting art because of the high permeability of sand generally, a characteristic which proves to be an advantage in most gravity casting situations. Under the pressure of die casting on the other hand, the porosity of the sand would allow air and other vapors from the molten metal to vent through the sand core, and this would be detrimental to the surface of the casting. This would tend to thwart one of the basic reasons for resorting to a pressure die casting process, namely that of achieving a close tolerance surface which is relatively free of imperfections. Thus it should be apparent that a packed sand core for use in carrying out the lost core process of the present invention is unlike the use of conventional sand cores in the foundry art generally. In order to prevent these gases from escaping into the core during the die casting process, a thin layer of impervious material is applied to the sand core in a manner to be described hereinbelow.

A packed sand core itself is a source of gas when subjected to the heat and pressure characteristic of a die casting machine. Since the cores used in this process cannot be adequately vented due to the very short time intervals involved in casting, it has been necessary to utilize a material which forms a minimum amount of gas. Sand cores made with a rather high percentage of silica and zirconium sand have proven more satisfactory than some of the cereal bound materials. Gypsum cores have also proven to be workable, and in this connection a hollow gypsum core molded by conventional slush-molding techniques has proven very successful in applications where the size of the hollow interior of the finished article permits such an approach. This technique produces an exceptionally fine avenue for the escape of core gas and permits the mixing of some cereal bound refractory material together with the gypsum in fabricating the lost core.

Having discussed the characteristics of the packed sand core to be used in practicing this second, or alternative, method of the invention, it will be appreciated that this type of core will benefit greatly from any method of reducing the formation of core gases. In this vein, the Vermiculite heat shield 18, discussed with reference to the first-mentioned method of practicing the lost core process above, represents a very effective way of achieving this end by insulating the core so as to reduce its temperature and thereby reduce the quantity of core gas liberated therefrom during the die casting process. It should be noted that the shielded or insulated core will reach its peak temperature only some time after the actual die casting step and thereby further enhance theadvantage set forth above. Additionally, as noted herei-nabove, some coating must be applied to the sand core in order to prevent mold gases from adversely affecting the surface of the die cast article. A packed sand core having a Vermiculite shield as described results in a sand core which is also impervious to these last-mentioned gases.

In summary then, the packed sand core is generally similar to the zinc core described in connection with the first method of practicing the invention. Both types of cores are of identical geometry for a given job application and both are provided with a heat resistant shield of Vermiculite or the like to impede heat transfer from the molten aluminum to the core material. These cores are similarly placed in the die casting machine as shown in FIG. 2 and the end portions 14 and 16 thereof function as supports for positioning the core during the actual casting step. Only the actual manner of destroying the packed sand core differs from the salt bath step outlined hereinabove in connection with the zinc core. In order to disintegrate the sand core, it is preferably crumbled by any suitable means which does not affect the hollow die cast article of FIG. 5. Among the various approaches which have been found successful are: vibration, shock, washing, and blowing out the core by air pressure or the like.

The invention claimed is:

1. The method of die casting a substantially hollow article and comprising the steps of providing a disposable core having an outer surface which has been treated to resist the transfer of heat thereacross, providing a die cover-half and a die ejector-half which engage to define a cavity for die casting said article, supporting said core in fixed relation to said die halves and said cavity at least when said halves are engaged, introducing molten metal into said cavity and around said core, said heat resistant outer surface of the core preventing influx of heat to the core as a result of contact between said core and said molten metal, thereafter removing said article and its associated core from the die ejector-half, subjecting said article and its associated core to a temperature high enough to melt said core by sustained application but not so high as to adversely affect said die cast material, said heat resistant core surface comprising a thin coating of Vermiculite flakes deposited on the core in a latex binder, which coating is incapable of maintaining the shape of the solidified core as the latter begins to melt, whereby melting of said core automatically destroys said heat resistant coating.

2. The method defined in claim 1 further characterized by said step of supporting said core in fixed relation to said die-halves being carried out by providing at least one core engageable cavity in said die-halves and by providing the core in a shape to cooperate with the lastmentioned cavity.

3. A core for use in a die casting machine having die ejector and cover halves which engage to define a cavity for casting a substantially hollow article, said core having an external surface which conforms generally to the shape of the hollow interior of said article, a heat resistant shield of Vermiculite flakes deposited on the core in a latex binder on said external core surface to deter the introduction of heat from the molten metal into said die cavity, and said heat shield being of such thickness that it is incapable of maintaining the shape of said external surface in the absence of the body of said core.

4. A core as defined in claim 3 wherein the body of said core has a melting point below the melting point of the metal from which said hollow article is to be cast, whereby said core can be substantially removed therefrom by heating said article and its associated core at a temperature above the melting point of said core.

5. A core as defined in claim 3 wherein the body of said core is made from packed sand and wherein said heat resistant Vermiculite shield thereon is bound in a latex material to a thickness which is impervious to gases generated in the core and at the same time thin enough to be incapable of maintaining the shape of the external surface of the core whereby said core can be substantially removed from said die cast article by crumbling.

6. The method of die casting a substantially hollow article and comprising the steps of providing a disposable core having an outer surface which has been treated to resist the transfer of heat thereacross, providing a die cover-half and a die ejector-half which engage to define a cavity for die casting said article, supporting said core in fixed relation to said die halves and said cavity at least when said halves are engaged, introducing molten metal into said cavity and around said core, said heat resistant outer surface of the core preventing influx of heat to the core as a result of contact between said core and said molten metal, thereafter removing said article and its associated core from the die ejector-half, crumbling said core, said core being made from packed sand, and said heat resistant core surface comprising a thin coating of Vermiculite flakes deposited on the core in a latex binder, said coating being thick enough to be impervious to gases generated in the core or in the cavity and thin enough to be incapable of maintaining the original shape of said packed sand core in the absence of the same, whereby crumbling of said co-re automatically destroys said heat resistant coating.

References Cited by the Examiner UNITED STATES PATENTS 1,901,124 3/1933 Saeger 22192 5 1,931,587 10/1933 McConnel 22-465 2,907,084 10/1959 Wood 22165 2,978,764 4/1961 Hrabovsky 22168 2,991,520 7/1961 Dalton 22165 FOREIGN PATENTS 10 1,034,549 4/1953 France.

99,269 7/1940 Sweden.

OTHER REFERENCES Die Casting, by Herb, first edition, 1936, pages 264 15 and 265.

Dietert: Modern Core Practices and Theories, pages 319-332, published 1942.

MARCUS U. LYONS, Primary Examiner. 2O

WILLIAM J. STEPHENSON, Examiner. 

1. THE METHOD OF DIE CASTING A SUBSTANTIALLY HOLLOW ARTICLE AND COMPRISING THE STEPS OF PROVIDING A DISPOSABE CORE HAVING AN OUTER SURFACE WHICH HAS BEEN TREATED TO RESIST THE TRANSFER OF HEAT THEREACROSS, PROVIDING A DIE COVER-HALF AND A DIE EJECTOR-HALF WHICH ENGAGE TO DEFINE A CAVITY FOR DIE CASTING SAID ARTICLE, SUPPORTING SAID CORE IN FIXED RELTION TO SAID DIE HALVES AND SAID CAVITY AT LEAST WHEN SAID HALVES ARE ENGAGED, INTRODUCING MOLTEN METAL INTO SAID CAVITY AND AROUND SAID CORE, SAID HEAT RESISTANT OUTER SURFACE OF THE CORE PREVENTING INFLUX OF HEAT TO THE CORE AS A RESULT OF CONTACT BETWEEN SAID CORE AND SAID MOLTEN METAL, THEREAFTER REMOVING SAID ARTICLE AND ITS ASSOCIATED CORE FROM THE DIE EJECTOR-HALF, SUBJECTING SAID ARTICLE AND ITS ASSOCIATED CORE TO A TEMPERATURE HIGH ENOUGH TO MELT SAID CORE BY SUSTAINED APPLICATION BUT NOT SO HIGH AS TO ADVERSELY EFFECT SAID DIE CAST MATERIAL, SAID HEAT RESISTANT CORE SURFACE COMPRISING A THIN COATING OF VERMICULITE FLAKE DEPOSITED ON THE CORE IN A LATEX BINDER, WHICH COATING IS INCAPABLE OF MAINTAINING THE SHAPE OF THE SOLIDIFIED CORE AS THE LATTER BEGINS TO MELT, WHEREBY MELTING OF SAID CORE AUTOMATICALLY DESTROYS SAID HEAT RESISTANT COATING. 